Racing Post has been hit with a ‘sophisticated, sustained and aggressive’ attack on 21 November and 22 November.
According to the horse racing daily site, hackers gained access to the database and stole details from each customer including usernames, first and last names, encrypted passwords, email and customer addresses and date of birth.
Racing Post editor Bruce Millington said security is an area the agency takes extremely seriously and the website has not been compromised previously.
"As soon as we were aware of the situation we did everything in our power to halt the breach," Millington said.
"As part of our efforts to resolve the issue, we have turned off the ability to register / log-on to racingpost.com.
"You will still be able to access the site safely. Members’ Club content will also be available.
"We are extremely sorry that this unfortunate incident has occurred."
Credit and debit card details remain safe as they have not been stored on the site during the attack.
Racing Post has also advised its users to take the safety measure of changing password on other sites if it is the same one being used for racingpost.com.
